Up to now, a radar of an FM-CW method using a milliwave band radio wave has been developed as a mobile radar. That is, the distance to a target and the relative speed to the target are determined in such a way that FM modulation is performed in a fixed frequency range around a fixed frequency in the center, a beat signal between a transmission signal and a reception signal is generated, and the beat frequency when the frequency of a transmission signal in the beat signal increases and the beat signal when the frequency of a transmission signal decreases are identified.
In order to identify the above-described up beat signal and down beat signal, the frequency spectrum of the beat signal is determined, and a fixed threshold value is set to separate signal components and noise components.
However, since the peak of beat signals in the frequency spectrum changes because of various causes, a signal and noise cannot be successfully separated only by simply setting a fixed threshold value. Accordingly, for example, a case in which a type of vehicle of a vehicle as a detection target is distinguished and a threshold value is set in accordance with the type of vehicle is disclosed in Patent Document 1.
Furthermore, a case in which a threshold value is set on the basis of a peak having the maximum power in accordance with the maximum power in a frequency spectrum changing when a rear vehicle suddenly appears in front of one's own vehicle is disclosed in Patent Document 2.
Furthermore, a case in which a threshold value is set on the basis of the power of a plurality of peaks to cope with a peak decreasing in accordance with the distance to a target is disclosed in Patent Document 3.
Moreover, a case in which a peak in a false echo image is eliminated by increasing a threshold value using the frequency of the false echo image generated by a harmonic, switching frequency, etc., is disclosed in Patent Document 4.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-214015
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-311260
Patent Document 3: Japanese Unexamined Patent Application Publication No. 4-318700
Patent Document 4: Japanese Unexamined Patent Application Publication No. 11-344560
In the radar shown in Patent Document 1, even if the type of vehicle is distinguished, since an actual signal strength changes from the existing state of things, noise cannot be accurately removed.
Furthermore, in the radars shown in Patent Documents 2 and 3, without setting a threshold value in a value and frequency range in accordance with each peak, a small target which is originally to be detected is lost.
Moreover, in the radar shown in Patent Document 4, there is a problem in that it is able to remove only the noise whose position of appearance can be predicted in advance.